1. Field of the Invention
The present invention relates to a method of separation/purification for high-purity silver chloride and a process for producing high-purity silver by the same, more specifically a method of separation/purification for high-purity silver chloride which, when a refining intermediate containing a sparingly soluble silver compound and one or more species of impurity elements is treated to separate high-purity silver chloride therefrom, needs no pretreatment step for the intermediate and which, when the high-purity silver is treated as a starting material to produce high-purity silver, can dispense with a reprocessing step by pyrometallurgical refining or electro-refining for metallic silver, and a process for producing high-purity silver by the same.
2. Description of the Prior Art
Silver has been traditionally recovered from anode slime discharged from an electro-refining step for smelting of copper, lead or the like. One of the methods widely used for separation/purification of high-purity silver from anode slime involves a pyrometallurgical treatment step for producing crude silver from the slime and pyrometallurgical refining or electro-refining step for treating the crude silver.
The pyrometallurgical treatment step involves environment-related problems caused by discharged dust and/or exhaust gases, and also problems related to working conditions, because it needs works in a hot atmosphere which may cause burn injuries. Under these situations, a hydrometallurgical process has been attracting attention for separation/recovery of silver.
For example, one method first converts a silver compound into a sparingly soluble compound, e.g., silver halide or sulfide, to preliminarily separate silver. It is widely used as one of the methods for separation/recovery of silver from an aqueous solution or solid by a hydrometallurgical process. This method can separate silver from most of concomitantly present metals, beginning with noble metals, e.g., gold and platinum group elements. However, the sparingly soluble compound is soluble to only a limited extent not only in water but also in an acidic or alkaline aqueous solution, and the separated silver compound is difficult to directly treat by the hydrometallurgical process to produce high-purity silver.
In order to solve the above problems, silver recovering methods in which a sparingly soluble silver compound produced from anode slime is treated as a staring materials, have been proposed. Some of the representative methods are described below. They involve their own problems.    (1) A method which leaches a starting material containing a sparingly soluble silver compound by an ammonia solution, and reduces the leaching liquor by a reducing agent (refer to, e.g., JP-A-2000-297332 (pages 1 and 2)). This method, although including no pretreatment step, gives a crude silver product containing a smaller quantity of impurities than the above-described pyrometallurgical process.
The problems involved in this method, however, are large consumption of ammonia by lead chloride, and need for an electro-refining step to produce high-purity silver, otherwise purity of silver it gives is limited to 99% or so.    (2) A method comprising several steps for leaching anode slime, which is treated to remove copper beforehand, in the presence of hydrochloric acid and hydrogen peroxide; treating the leaching residue with sodium carbonate to covert lead chloride into the carbonate; ammonia-aided leaching to extract silver chloride into a solution; neutralization with sulfuric acid to precipitate silver chloride; chloro-scrubbing of the precipitation with hydrochloric acid and hydrogen peroxide; treating the chloro-scrubbing residue with sodium hydroxide to convert silver chloride into silver oxide, and reducing the silver oxide into a high-quality product by reducing silver with a reducing agent, e.g., a saccharide having a reducing capability, hydrazine monohydrate (refer to, e.g., JP-B-3,086,655 (pages 1 and 2)).
One of the problems involved in this method, however, is need for a lead-removing step, proposed to avoid consumption of ammonia by lead chloride in the ammonia-aided leaching step. Therefore, it additionally needs an agent for removing lead, and hence a process comprising leaching, filtration, decomposition of the leaching liquor and filtration. Moreover, it additionally needs a step for treating the separated lead compound. Still more, it is difficult for this method to recover silver having a purity exceeding 99.99%.    (3) A method which recovers silver from a starting material containing one or more species of noble metals including silver, and lead. It comprises several steps for leaching the starting material by chloridation to form a precipitation containing silver and lead chlorides and solid-liquid separation to recover the precipitation containing silver and lead chlorides; repulping the precipitation containing silver and lead chlorides with water and reducing the slurry with powdered iron to recover a mixture containing metallic silver and lead; oxidation of the mixture containing silver and lead at high temperature in a dry furnace to form slag containing crude silver and lead oxide; and separation of crude silver and electro-refining of crude silver into high-purity silver (refer to, e.g., JP-A-2001-316736 (pages 1 and 2)).
One of the problems involved in this method, however, is that it basically shows no improvement in solving the above problems inherent in the pyrometallurgical process.    (4) A method which comprises several steps for pretreatment of a chlorine-aided leaching residue containing silver chloride as a major component with a 40 to 80 g/L NaOH solution; leaching the residue with a 80 to 150 g/L sodium thiosulfate solution to produce the liquor containing silver; extraction of the liquor with a 0.1 to 1M/L tricapryl methylammonium salt solution to separate silver; stripping of the extract with a 1 to 2.5M/L alkaline nitrate solution; and reducing the strip liquor to produce metallic silver (refer to, e.g., JP-A-2003-105456 (pages 1 and 2)).
One of the problems involved in this method, however, is need for a large-size system and hence high investment cost, because of low silver concentration of the silver strip liquor. Another problem is difficulty in increasing silver purity, because of lack of purification step for strip liquor.
As discussed above, the methods proposed so far for separating silver by leaching from a sparingly soluble silver compound, e.g., silver chloride, in the presence of an ammonia or sodium thiosulfate solution have a common problem that they need a reprocessing step by pyrometallugical refining or electro-refining for silver recovered by reduction, because many impurity metals which can form a complex with ammonia or sodium thiosulfate are simultaneously leached in the leaching step.
When ammonia is used, the silver ion may react with ammonia while being settled to form an unstable, explosive silver compound, e.g., fulminating silver, which poses an obstacle to liquid storage or recycling. When sodium thiosulfate is used, on the other hand, silver sulfide tends to precipitate, while the solution is stored, the reaction being notably accelerated when the solution decreases in pH level or increases in temperature. This may increase concentration of sulfur present in metallic silver, when it is recovered by electrolysis or reduction.
It is also known that an organophosphorus compound, e.g., alkyl phosphine sulfide, can selectively extract silver as one of other methods for separating an impurity element. However, it cannot be used for separation by solvent extraction from leaching liquor produced by leaching with a thiosulfate, because silver reacts with the thiosulfate ion to form a stable complex.
Under these situations, there are demands for separation/purification methods which efficiently produce high-purity silver chloride and silver from a refining intermediate containing a sparingly soluble silver compound and one or more species of impurity elements without needing a pretreatment step for the intermediate and reprocessing step for recovered metallic silver by pyrometallurgical refining or electro-refining.